VI PENEIDEA AND CARIDEA SHRIMPS AND PRAWNS I 59 



are described by Alcock ^ as possessing peculiar secondary sexual 

 characters. Thus Farapeneus rectacutus 6 has one lash of the 

 first pair of antennae peculiarly bent to form a clasping organ, 

 while Aristaeus crasslj^es has a hook on the end of the third 

 niaxillipede. In the latter the females have much longer rostra 

 than the males, and are in general more powerfully built, so that 

 they seem to have usurped the proper functions of the male, and 

 probably engage in combats with one another over his person. 



As a general rule the Shrimps and Prawns occur in large 

 shoals in the shallow waters of the littoral zone, and they have 

 a remarkable power of adapting their colours to the surroundings 

 in which they happen to be at any particular moment.^ This is 

 brought about by the variously coloured chromatophores, wliich 

 contract and expand 

 in obedience to a 

 stimulus transmitted 

 through the eyes of the 

 animal. A number of 

 the Palaemonidae go 

 up rivers into fresh 

 water, while one 

 family, the Atyidae, 

 live in the completely 

 fresh water of rivers 

 and inland lakes. The 

 Peneidea undergo a 

 very complete meta- 

 morphosis which is 

 primitive in respect 

 to the order of forma- 

 tion of the segments 

 from before backwards. 

 The larva hatches out 

 as a Nauplius (Fig. lOG), which by tlie orderly addition of segments 



Fig. 106. — Nauplius larva of Penens, sp. x 25. 

 (From Balfour, after F. Jliiller). 



^ Loc. cit. p. 150. 



- Keeble and Gamble, Phil. Trans., Ser. B, cxcvi., 1904, p. 295. The chromato- 

 phores are also directly responsive to light, but the lasting adaptations to colour- 

 backgrounds are brought about indirectly, the stimulus being transmitted through 

 the eyes and nervous system. The influence of light may also atfect the metabolism 

 of the animal, the chromatophores being accompanied l)y a ramifying fatty tissue, 

 wiiich disappears if the animal is kept in the dark. 



